Applications for industrial ion and plasma sources include etching, deposition and property modification, as described by Kaufman, et al., in the brochure entitled Characteristics, Capabilities, and Applications of Broad-Beam Sources, Commonwealth Scientific Corporation, Alexandria, Va. (1987).
Both gridded and gridless ion sources are used in these industrial applications. Gridded ion sources are described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745. The end-Hall ion source is one type of gridless ion source and is described in U.S. Pat. No. 4,862,032—Kaufman, et al., while the closed-drift ion source is another type of gridless ion source and is described by Zhurin, et al., in an article in Plasma Sources Science & Technology, Vol. 8, beginning on page R1. These publications are incorporated herein by reference.
Both gridded and gridless ion sources incorporate electron-emitting hot-filament cathodes. These cathodes function as cathode-neutralizers in gridless ion sources and as both discharge-chamber cathodes and neutralizers in gridded ion sources. Power supplies provide heating currents for these cathodes, where the heating current increases the cathode temperature sufficiently for the thermionic emission of electrons. Some of the materials, operating conditions, common problems, and lifetime limitations of electron-emitting hot-filament cathodes are described by Kaufman, et al., in Chapter 3 of the book, Operation of Broad-Beam Sources, Commonwealth Scientific Corporation, Alexandria, Va. (1984).
The design techniques for these power supplies employ a conversion frequency to convert from line voltage and current to output voltage and current. This conversion frequency is either 50-60 Hz or a considerably higher frequency, typically ≧25 Khz. The 50-60 Hz techniques are called “linear/line-frequency-phase-control” (or “linear/LFPC”) herein. The techniques in which a conversion frequency independent of line frequency is generated and digital on-off states are used for the power switching devices are called “switch-mode.” Examples of switch-mode techniques are given by Pressman in the book, Switching Power Supply Design, McGraw-Hill, Inc., New York (1991).
Power supplies based on linear/LFPC have been readily available for many years, but are large and heavy, due mostly to the large, heavy 50-60 Hz transformers that are incorporated. Power supplies based on switch-mode techniques have been readily available for the last two decades and are characterized by smaller sizes and lighter weights, due to the much smaller and lighter transformers that can be used at the higher frequencies.
The heating current generated by the power supplies is of two types: direct current (dc) and alternating current (ac) Either linear/LFPC or switch-mode power supplies can be used to generate a dc heating current for a hot-filament cathode. With either type of power supply, however, the cathode lifetime is substantially reduced due to the dc nature of the current.
Either linear/LFPC or switch-mode power supplies can also be used to generate an ac heating current. In either case the power output from a well-designed supply is usually from an output transformer operating at the conversion frequency that is used, i.e., either 50-60 Hz or ≧25 Khz. While the use of a 50-60 Hz output frequency presents no problems, the reactive impedances in the transmission line between the power supply and the cathode are significant at the ≧25 Khz frequency. These reactive impedances cause impedance matching problems. An important exception to the use of an output transformer is described in U.S. Pat. No. 6,911,789—Geissler, et al, which is incorporated herein by reference.
The use of an output transformer permits the connection to a reference potential to be made through a center tap in the transformer secondary to both minimize plasma disturbances and increase filament lifetime. In the discharge chamber of a gridded ion source, the reference potential is the negative terminal of the discharge supply. In the neutralizer of a gridded ion source or the cathode-neutralizer of a gridless ion source, the reference potential is usually ground. If a discharge is used to generate a plasma that fills a vacuum chamber, the reference potential would usually be the potential of the vacuum chamber.
To summarize the technology, linear/LFPC power supplies that generate 50-60 Hz heating currents are widely used, but are large and heavy due to the transformers used in them. Switch-mode power supplies that generate ac heating currents at a ≧25 Khz conversion frequency are much smaller and lighter, but are limited by impedance and impedance-matching effects of the transmission line between the power supply and the hot-filament cathode.